For the electron tubes such as image intensifiers, there has been heretofore a need for a technology to prevent degradation of withstand voltage characteristics. The reason for it is that the degradation of withstand voltage characteristics of the electron tube would lead to a local discharging phenomenon and in turn to degradation of output characteristics of the electron tube. For suppressing this discharging phenomenon, for example, the electron tube described in Patent Literature 1 adopts a structure in which a part of an envelope is covered by a chromium oxide layer. Furthermore, for example in the electron tubes described in Patent Literatures 2 and 3, an electrically insulating film comprised of Al2O3, ZnO, and others is formed on an inner wall surface of a metal housing or on a neck portion of a glass housing.